That said, XML tunings and the like are less of a problem since the Pets-related updates (I think it was). If you look in the Gameplay Data, you'll see that the resources have been split up for the various EP's and they're already there for every game.
The worst that a Tuning Mod, for example, can do to a game that doesn't have the required EP is change the variables for something that doesn't get used anyway.
Fyi, you probably should have downloaded the expansion before downloading a ton of mods. Before you do install it, take all of your mods out, install it, then go through to see what you have. If you think it probably won't work with the new expansion, don't put it back in.
Also, if you plan on patching your game at all, know that many mods are also patch specific on top of being EP specific.
If you remove a modded object (or any custom object for that matter) to update an EP, and you have any lots with that object in, it's a good idea to not play that neighborhood again until you have updated all the modded objects. Otherwise you could either corrupt your lot, or end up with the objects missing from your lots and having to refurnish them.
Please do not PM me with questions about modding. Please post in an appropriate forum and send me a link to the thread if you would like me to try and help.
Even base game mods should be updated to the current patch level of the game. If you have a fully patched game, and have up-to-date mods, you should have no problem at all when adding an expansion. If you have an unpatched (or not fully patched) base game, you need to check to which base game patch level you're patched after adding the expansion and update your mods to that level.
However, it's not always easy or possible to find mods for specific older patch levels, so I'd recommend always patching to the latest level and keeping your mods up to date.